Lithium ion secondary battery including an elastic member arranged inside an electrode assembly and method for producing the same

ABSTRACT

A lithium ion secondary battery includes: an electrode assembly including a positive electrode plate, a negative electrode plate, and a separator; and an elastic member. The electrode assembly is wound around an axis into a flat shape having a curved end defining a corner portion. The elastic member is arranged inside the electrode assembly and into at least the corner portion of the electrode assembly such that at least a portion of the elastic member is positioned between at least a portion of one of the positive electrode plate and the negative electrode plate and at least a portion of the separator in a direction perpendicular to the axis. The elastic member is made of a material having porosity.

RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application No. PCT/JP2011/000102, filed on Jan. 12, 2011,which in turn claims the benefit of Japanese Application No.2010-005215, filed on Jan. 13, 2010, the disclosures of whichApplications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a lithium ion secondary battery havinga flat shape and a method for producing the battery.

BACKGROUND ART

As a typical structure of lithium ion secondary batteries, there hasbeen known a so-called “wound structure” in which an electrode assemblyformed by combining a positive electrode plate, a negative electrodeplate, and a separator is spirally wound. Among them, lithium ionsecondary batteries in which such an electrode assembly is wound into aflat shape are much in demand in the field of mobile devices, etc. Inorder to prevent the wound electrode assembly from being loosened, thewinding end portion of the electrode assembly is usually fixed with anadhesive member (typically an adhesive tape).

Meanwhile, a phenomenon that an electrode assembly expands as a batteryis charged is known to those skilled in the field of lithium ionsecondary batteries. Specifically, a positive electrode plate expands involume as a positive electrode active material is dedoped with lithiumions, and a negative electrode plate expands in volume as a negativeelectrode active material is doped with lithium ions. However, since thewinding end portion is fixed with an adhesive member, the outwardexpansion of the electrode assembly is restricted. Therefore, arelatively large stress is generated in the electrode assembly, and theelectrode assembly may be deformed to release this stress. For example,the electrode assembly is deformed like folded strata in some cases. Thedeformation of the electrode assembly often accompanies an increase inthe thickness of the battery.

In order to address this problem, JP 2006-302801 A describes a techniquefor fixing the winding end portion with an adhesive member provided witha folded portion in a part of the substrate where no adhesive ispresent. With the use of the adhesive member described in this patentliterature, the folded portion is unfolded and thereby allows thewinding of the electrode assembly to be loosened. This means that aspace is secured to absorb the expansion of the electrode assembly evenafter it is wound. This can suppress the increase in the thickness ofthe battery during charging.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2006-302801 A

SUMMARY OF INVENTION Technical Problem

According to the technique described in JP 2006-302801 A, the effect ofsuppressing an increase in thickness can be achieved to some extent infact, but that effect is not necessarily sufficient. Under thesecircumstances, it is an object of the present invention to provide afurther improved technique for suppressing an increase in the thicknessof a battery during charging.

Solution to Problem

The present invention provides a lithium ion secondary battery having aflat shape. This battery includes:

an electrode assembly including a positive electrode plate, a negativeelectrode plate, and a separator, the electrode assembly being woundinto a flat shape; and

an elastic member incorporated inside an outermost of the electrodeassembly and into a corner portion of the electrode assembly in awinding direction, the elastic member having porosity.

In another aspect, the present invention provides a method for producinga lithium ion secondary battery having a flat shape. This methodincludes:

a step of preparing a positive electrode plate, a separator, and anegative electrode plate;

a step of forming an electrode assembly by combining the positiveelectrode plate, the separator, and the negative electrode plate, andwinding the electrode assembly;

a step of placing an elastic member having porosity on the positiveelectrode plate, the negative electrode plate, or the separator beforethe end of the winding step so that the elastic member is incorporatedinside an outermost of the electrode assembly and into a corner portionof the electrode assembly in a winding direction; and

a step of fixing a winding end portion of the electrode assembly.

Advantageous Effects of Invention

According to the above method of the present invention, the porouselastic member is incorporated inside the outermost of the electrodeassembly and into the corner portion of the electrode assembly in thewinding direction. When the electrode assembly expands, the porouselastic member is subjected to load from the electrode assembly andreduces its volume. The elastic member reduces its volume and therebyallows the electrode assembly to expand. In particular, with such anelastic member incorporated in the corner portion, the electrodeassembly is allowed to expand in the in-plane direction (in thedirection perpendicular to the thickness direction) of the battery. As aresult, the increase in the thickness of the battery during charging canbe effectively suppressed, and the initial thickness of the battery canbe reduced.

In this description, the “initial thickness of a battery” means thethickness of the battery that has been charged for the first time afterthe assembly thereof.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a lithium ion secondary batteryaccording to one embodiment of the present invention.

FIG. 2 is a partial cross-sectional view of an electrode assembly usedin the lithium ion secondary battery shown in FIG. 1.

FIG. 3 is a cross-sectional view of the outermost of the electrodeassembly.

FIG. 4 is a plan view showing the position where an elastic membershould be placed.

FIG. 5 is an exploded perspective view of another lithium ion secondarybattery using an aluminum battery case.

FIG. 6 is a flow chart for the production of the lithium ion secondarybattery having the electrode assembly shown in FIG. 3.

FIG. 7 is a schematic diagram showing the steps of forming a woundelectrode assembly.

FIG. 8 is a perspective view showing another position where the elasticmember should be placed.

FIG. 9 is a cross-sectional view of another electrode assembly in whichapproximately one turn of the elastic member is incorporated.

FIG. 10 is a plan view and a cross-sectional view of an adhesive memberdescribed in JP 2006-302801 A.

DESCRIPTION OF EMBODIMENTS

As shown in FIG. 1, a lithium ion secondary battery 10 of the presentembodiment includes a battery case 11 and an electrode assembly 30accommodated in the battery case 11. The battery 10 has a flat,rectangular shape. The electrode assembly 30 also has a flat,rectangular shape. The electrode assembly 30 is provided with a positiveelectrode lead 33 and a negative electrode lead 34. The leads 33 and 34are drawn out of the battery case 11.

As shown in FIG. 2, the electrode assembly 30 is formed by combining apositive electrode plate 14, a negative electrode plate 18, andseparators 15. The positive electrode plate 14 is formed of a positiveelectrode current collector 12 and positive electrode active materiallayers 13 provided on both sides of the positive electrode currentcollector 12. The negative electrode plate 18 is formed of a negativeelectrode current collector 16 and negative electrode active materiallayers 17 provided on both sides of the negative electrode currentcollector 16. The separator 15 is disposed between the positiveelectrode plate 14 and the negative electrode plate 18. Two or more(typically two) separators 15 are used to form the wound electrodeassembly 30. The electrode assembly 30 is impregnated with a non-aqueouselectrolytic solution.

As shown in FIG. 3, the electrode assembly 30 is spirally wound. Porouselastic members 21 are incorporated inside the outermost of the woundelectrode assembly 30 and into corner portions 31 of the electrodeassembly 30 in the winding direction. The elastic members 21 function toabsorb the expansion of the electrode assembly 30 during charging. Theoutermost of the electrode assembly 30 includes a winding end portion 32as the end of the winding. The winding end portion 32 is formed of thepositive electrode current collector 12, and is fixed to the positiveelectrode current collector 12 itself with an adhesive tape 19 (anadhesive member). The “corner portions 31” mean arc-shaped portions ofthe wound electrode assembly 30. The “winding direction” means adirection parallel to the longitudinal direction of the unwoundelectrode assembly 30 and perpendicular to the thickness direction ofthe battery 10.

As described later in a modification, the elastic member 21 may beincorporated into a portion other than the corner portions 31. However,if the elastic members 21 are incorporated only into the corner portions31 as in the present embodiment, an increase in the thickness of theelectrode assembly 30 by the elastic members 21 themselves can beavoided. Furthermore, even if the elastic members 21 are incorporatedonly into the corner portions 31, the effect of suppressing an increasein the thickness of the battery 10 during charging can be sufficientlyobtained.

The electrode assembly 30 has the corner portions 31 that are formed attwo positions in the winding direction. In the present embodiment, theelastic member 21 is incorporated into each of the two corner portions31. With this configuration, the effect of suppressing the deformationof the electrode assembly 30 is obtained uniformly in the in-planedirection of the battery 10. The effect of suppressing the increase inthe thickness of the battery 10 also is expected to be enhanced. Even ifthe elastic member 21 is incorporated into only one of the cornerportions 31, the effect of suppressing the increase in the thickness canbe obtained to some extent, of course.

In the present embodiment, the outermost of the electrode assembly 30consists of the positive electrode current collector 12. That is, theoutermost includes the positive electrode current collector 12 withoutthe positive electrode active material layer 13 provided thereon. Thewinding end portion 32 also consists of the positive electrode currentcollector 12. The elastic member 21 is placed in a portion of thepositive electrode plate 14 consisting of the positive electrode currentcollector 12. Specifically, the elastic member 21 is placed between aportion of the positive electrode current collector 12 that constitutesthe outermost and a portion of the positive electrode current collector12 located just (one turn) inside the outermost. The positive electrodeactive material layer 13 is provided on only one side of the positiveelectrode current collector 12 located just inside the outermost. If theelastic member 21 is placed in the region where the positive electrodeactive material layer 13 is not provided, as described above, it ispossible to prevent the production of an active material that does notcontribute to the power generation.

If the positional relationship between the positive electrode plate 14and the negative electrode plate 18 is reversed, the outermost of theelectrode assembly 30 can consist of the negative electrode currentcollector 16, although not shown in the drawings. In this case, theelastic member 21 can be placed between a portion of the negativeelectrode current collector 16 that constitutes the outermost and aportion of the negative electrode current collector 16 located just (oneturn) inside the outermost. Furthermore, in the present embodiment,approximately one turn of the portion consisting of the positiveelectrode current collector 12 is formed. However, even if the portionconsisting of the positive electrode current collector 12 or thenegative electrode current collector 16 is formed longer than one turn,this portion is included in the concept of the “outermost”. In otherwords, in the “outermost”, the positive electrode active material layerand the negative electrode active material layer do not face each other.

The following effects are obtained by placing the elastic member 21 onthe positive electrode current collector 12 without the positiveelectrode active material layer 13 provided thereon. First, since thepositive electrode active material layer 13 is not provided, the size ofthe electrode assembly 30 can be reduced. Furthermore, the elasticmember 21 can be attached easily with an adhesive agent, an adhesivetape, or the like, to the surface of the positive electrode currentcollector 12 without the positive electrode active material layer 13provided thereon. These effects can also be obtained as well in the casewhere the outermost consists of the negative electrode current collector16.

Meanwhile, also in the case where the space is formed inside the portionother than the outermost of the electrode assembly 30, the effect ofabsorbing the expansion of the electrode assembly 30 can be obtained. Asan example, the present inventor conducted the following preliminaryexperiment.

First, spacers (first spacers) having a thickness of 45 μm and a widthof 10 mm were prepared. Next, a wound electrode assembly with 8 turnswas fabricated by the method described later with reference to FIG. 6.During the winding of the electrode assembly, the first spacers wereinserted into the corner portions in each winding turn. That is, 16first spacers were used in total. After the winding step, the firstspacers were pulled out of the electrode assembly. Thereby, in theresulting electrode assembly, spaces resulting from the first spacerswere formed in the corner portions in each winding turn. The electrodeassembly thus obtained was accommodated in a battery case.

Meanwhile, spacers (second spacers) having a thickness of 360 μm and awidth of 10 mm were prepared. Next, a wound electrode assembly with 8turns was fabricated by the method described later with reference toFIG. 6. When the electrode assembly was wound, the second spacers wererespectively inserted into the two corner portions formed inside theoutermost. After the winding step, the second spacers were pulled out ofthe electrode assembly. Thereby, in the resulting electrode assembly,spaces resulting from the second spacers were formed in the two cornerportions inside the outermost. The electrode assembly thus obtained wasaccommodated in a battery case.

The initial thicknesses of these two types of batteries thus produced bythe above methods were measured. Both of them had a thickness of 5.51mm. This result of the preliminary experiment shows that wherever in theelectrode assembly the space is formed, the effect of suppressing theincrease in the initial thickness can be obtained.

If this is true, it is most advantageous to incorporate the elasticmember 21 inside the outermost. This is because if the elastic member 21is placed on the active material layer 13 or 17 in the electrodeassembly 30, the active material covered with the elastic member 21 iswasted without being used. The portion where the active material layer13 or 17 is not provided may be formed in the electrode assembly 30, butthe steps for forming the active layers 13 and 17 on the currentcollectors 12 and 16 may be complicated. The capacity of the batteryalso may decrease.

The elastic member 21 can be made of a material having porosity. If theelastic material 21 is made of a material having porosity, the expansionof the electrode assembly 30 can be absorbed by changes in the shape andvolume of the elastic member 21 itself. Instead, a material like rubberhaving elasticity but not having porosity shows little change in volumewhen it is subjected to load. Therefore, such a material is of littleuse as a material for the elastic member 21 of the present invention.Preferably, the elastic member 21 has insulation properties, andtypically, it can be made of a resin material.

Specifically, the elastic member 21 can be made of at least one materialselected from the group consisting of polyethylene, polypropylene, andpolyurethane. These materials are preferred because they do not dissolvein electrolytic solutions commonly used for lithium ion secondarybatteries and do not affect the battery characteristics. Morespecifically, the elastic member 21 can be formed of a foam of any ofthe above materials. The foam of any of the above materials changes itsshape and volume when it is subjected to load. This means that such afoam satisfies the properties required by the elastic member 21. Eitheropen-cell foams or closed-cell foams can be used.

It should be noted that the “elasticity” required by the elastic member21 does not necessarily mean rubber elasticity. It means an elasticityhigh enough for the elastic member 21 to change its shape and volume andthereby allow the electrode assembly 30 to expand. For example, amaterial that undergoes a volume change of 10% or more under a pressureof 10 MPa at room temperature can be used for the elastic member 21.Furthermore, if the elastic member 21 has the property of easilyreturning to its original shape after it is deformed, it is possible toconvey the elastic member 21 by nipping it with rolls or the like in theproduction process of the battery 10.

There is no particular limitation on the dimensions of the elasticmember 21. The dimensions should be adjusted as appropriate according tothe design of the electrode assembly 30 into which the elastic member 21should be incorporated. As shown in FIG. 4, the dimension W₁ of theelastic member 21 can be adjusted, for example, to the range of thedimension W₂ of the positive electrode current collector 12 or greaterand the dimension W₃ of the separator 15 or smaller in the widthdirection WD of the (unwound) electrode assembly 30. That is, theelastic member 21 is provided to extend across the positive electrodecurrent collector 12 in the width direction WD. With this configuration,the expansion of the electrode assembly 30 in the in-plane direction ofthe battery 10 can be absorbed uniformly. The elastic member 21 has apredetermined length L₁ in the longitudinal direction LD of theelectrode assembly 30. This predetermined length L₁ is adjusted, forexample, within the range in which the elastic member 21 does notincrease the thickness of the battery 10 when it is incorporated onlyinto the corner portion 31. There is also no particular limitation onthe shape of the elastic member 21. Various shapes such as plate-like,square bar, and round bar shapes can be employed. Furthermore, theelastic member 21 may be previously formed into a shape fitted in thecorner portion 31, for example, a shape having a crescent-shaped crosssection.

In FIG. 3, the separator 15 ends just short of the corner portion 31,but it may extend into the corner portion 31 in some cases. That is, itmay be possible for the end portion of the separator 15 to constitute apart or the whole of the elastic member 21. On the other hand, in thepresent embodiment, the elastic member 21 is formed of a memberdifferent from the separator 15. In this case, the elastic member 21 hasa thickness clearly exceeding that of the separator 15. It should benoted that the thickness of the elastic member 21 is identified by thedimension of the elastic member 21 in the thickness direction of theelectrode assembly 30 in the unwound state.

The elastic members 21 as described above are incorporated in theelectrode assembly 30. The elastic members 21 contract themselves andthereby can absorb the volume expansion of the positive electrode plate14 caused by dedoping of lithium ions from the positive electrode activematerial and the volume expansion of the negative electrode plate 18caused by doping of lithium ions into the negative electrode activematerial. The electrode assembly 30 is allowed to expand as theelectrode plates expand in volume during charging. Therefore, not onlythe deformation of the electrode assembly 30 can be suppressed but alsothe increase in the thickness of the battery 10 can be suppressed.

Next, other components of the battery 10 are described individually.

The positive electrode current collector 12 is made of a metal foil,typically an aluminum foil. The metal foil may be subjected to lathingor etching. The positive electrode active material layer 13 contains apositive electrode active material, a binder, and a conductive agent.The thickness of the positive electrode plate 14 is in the range inwhich its sufficient flexibility can be secured, for example, in therange of 50 to 200 μm.

The positive electrode active material is not particularly limited aslong as it can occlude and release lithium ions. For example, alithium-containing transition metal compound can be used. Examples ofthe lithium-containing transition metal compound include composite metaloxides containing lithium and at least one metal selected from the groupconsisting of cobalt, manganese, nickel, chromium, iron, and vanadium.Examples of such composite metal oxides containing lithium includeLiCoO₂, LiMnO₂, LiNiO₂, LiCo_(x)Ni_((1-x))O₂ (0<x<1), LiCrO₂, αLiFeO₂,and LiVO₂.

Examples of binders that can be used include fluororesin, acrylicrubber, modified acrylic rubber, styrene-butadiene rubber, acrylicresin, and vinyl resin. These binders may be used alone, or two or moreof them may be used in combination. Furthermore, copolymers of two ormore monomers used for the resins listed above may be used as binders.Examples of fluorine-containing binders include polyvinylidene fluoride,vinylidene fluoride-hexafluoropropylene copolymer, andpolytetrafluoroethylene.

As the conductive agent, carbon materials such as acetylene black,graphite, and carbon fiber can be used. These carbon materials may beused alone, or two or more of them may be used in combination.

As the negative electrode current collector 16, a copper foil or acopper alloy foil can be suitably used, but it is not limited to these.Specific examples of such foils include a rolled foil and anelectrolytic foil. The shape of the foil is not particularly limited.The foil may be a punched foil, an expanded material, a lath material,or the like. A previously roughened electrolytic copper foil orroughened rolled copper foil also can be used.

The negative electrode active material layer 17 contains a negativeelectrode active material as a main component (whose content is highestin mass ratio), and further contains a binder and/or a conductive agentas an optional component. The thickness of the negative electrode plate18 is in the range in which its sufficient flexibility can be secured,for example, in the range of 50 to 210 μm.

As the negative electrode active material, a material capable ofreversibly occluding and releasing lithium ions can be used. Forexample, a material containing graphite having a graphite-type crystalstructure can be used. Examples of such materials include naturalgraphite, spherical or fibrous artificial graphite, non-graphitizablecarbon (hard carbon), and graphitizable carbon (soft carbon). Silicon,tin, silicon-tin alloys, oxides such as silicon oxide and tin oxide,nitrides such as silicon nitride also can be suitably used to obtain ahigh energy density.

The optional additives such as a binder, a conductive agent, and athickener used for the positive electrode active material layer 13 alsocan be used for the negative electrode active material layer 17.

Examples of the separator 15 include microporous polyolefin membranessuch as a microporous polyethylene membrane and a microporouspolypropylene membrane. These can be used as a single layer membrane ora multilayer membrane in which two or more of these are stacked. Forexample, a multilayer membrane in which microporous polypropylenemembranes are stacked on both sides of a microporous polyethylenemembrane can be used as the separator 15. The separator 15 has athickness in the range of 8 to 30 μm, for example.

The electrolytic solution can be prepared by dissolving an electrolytein a non-aqueous solvent. Examples of the non-aqueous solvent that canbe used include ethylene carbonate, propylene carbonate, butylenecarbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone,1,2-dimethoxyethane, 1,2-dichloroethane, 1,3-dimethoxypropane,4-methyl-2-pentanone, 1,4-dioxane, acetonitrile, propionitrile,butyronitrile, valeronitrile, benzonitrile, sulfolane,3-methyl-sulfolane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylsulfoxide, dimethylformamide, trimethyl phosphate, and triethylphosphate. These non-aqueous solvents can be used alone, or as a mixedsolvent of two or more of them.

As the electrolyte, for example, strongly electron-attracting lithiumsalts can be used. Specific examples of such lithium salts includeLiPF₆, LiBF₄, LiClO₄, LiAsF₆, LiCF₃SO₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, andLiC(SO₂CF₃)₃. These electrolytes may be used alone, or two or more ofthem may be used in combination. These electrolytes are dissolved in anon-aqueous solvent so that the concentrations thereof are in the rangeof 0.5 to 1.8 mol/liter, for example.

The materials for the adhesive tape 19 are not particularly limited aslong as they are not dissolved or decomposed in non-aqueous electrolyticsolutions. Examples of the adhesive agent for the adhesive tape 19include a partially crosslinked copolymer of a monomer of an acrylicacid alkyl ester such as butyl acrylate and a monomer such ashydroxyethyl acrylate. Examples of the substrate for the adhesive tape19 include unstretched or stretched films made of resins such aspolyethylene terephthalate, polyphenylene sulfide, polypropylene,polystyrene, polycarbonate, and polymethylmethacrylate.

As the battery case 11, a so-called laminated packaging material, apackaging material obtained by laminating a metal foil such as analuminum foil with a resin such as polyethylene terephthalate, can beused. A laminated packaging material is advantageous in reducing theweight and thickness of the battery, but on the other hand, it hasflexibility and is susceptible to the effect of the deformation of theelectrode assembly 30. Therefore, if the present invention is applied toa lithium ion secondary battery using a laminated packaging material, ahigher effect can be obtained.

Alternatively, as shown in FIG. 5, the electrode assembly 30 may beaccommodated in a battery case 41 made of a hard material instead of alaminated packaging material. In this case, from the viewpoint ofcompressive strength, an aluminum alloy containing trace amounts ofmetals such as manganese and copper or a steel sheet plated with nickelcan be suitably used as a material for the battery case 41. The batterycase 41 includes a bottomed case body 42 with an open upper end and asealing plate 43 for sealing the opening of the case body 42. A negativeelectrode lead and a positive electrode lead are connected electricallyto the polar terminal of the sealing plate 43 and the polar terminal ofthe case body 42 (or the portion other than the negative electrodeterminal of the sealing plate 43), respectively. The sealing plate 43 islaser-welded to the case body 42, and then a non-aqueous electrolyticsolution is poured into the battery case 41 through a pouring port (notshown) provided on the sealing plate 43. Then, a stopper (not shown) isput on the pouring port and the port is sealed by laser welding.

Next, the method for producing a lithium ion secondary battery isdescribed with reference to the flow chart shown in FIG. 6.

First, the positive electrode plate 14, the separator 15, and thenegative electrode plate 18 are prepared (Step S1). The positiveelectrode plate 14 can be fabricated by applying a positive electrodematerial mixture to the strip-shaped positive electrode currentcollector 12, followed by drying and rolling. The positive electrodematerial mixture can be prepared by mixing a positive electrode activematerial, a binder, and a conductive agent with a suitable dispersionmedium. The positive electrode active material layer 13 can be formed byapplying the positive electrode material mixture on one side or bothsides of the positive electrode current collector 12, followed by dryingand rolling.

As the dispersion medium, those capable of dissolving the binder aresuitable. Specifically, organic solvents such as N-methyl-2-pyrrolidone,N,N dimethylformamide, tetrahydrofuran, dimethylacetamide, dimethylsulfoxide, hexamethylsulfonamide, tetramethylurea, acetone, and methylethyl ketone can be suitably used. These organic solvents may be usedalone or two or more of them may be used in combination. Furthermore,water or hot water may be used as the dispersion medium as long as itcan dissolve the binder.

Additives such as a dispersant, a surfactant, and a stabilizer may beadded to the positive electrode material mixture, as necessary.Furthermore, a thickener such as an ethylene-vinyl alcohol copolymer,carboxymethyl cellulose, and methyl cellulose may be added to thepositive electrode material mixture, as necessary.

The method for applying the positive electrode material mixture to thepositive electrode current collector 12 is not particularly limited. Thepositive electrode material mixture can be easily applied with, forexample, a slit die coater, a reverse roll coater, a lip coater, a bladecoater, a knife coater, a gravure coater, a dip coater, or the like. Themethod for drying the applied positive electrode material mixture alsois not particularly limited. Either natural drying or heat drying can beemployed. In view of productivity, it is recommended to employ a dryingmethod in which the positive electrode material mixture is dried at anambient temperature of 70 to 200° C. for 10 minutes to 5 hours, forexample. Rolling can be performed with a roll press machine. Rolling maybe performed a few times or with various pressures applied so that thepositive electrode active material layer 13 with a predeterminedthickness is formed.

The negative electrode plate 18 can also be fabricated by applying anegative electrode material mixture to the strip-shaped negativeelectrode current collector 16, followed by drying and rolling, in thesame manner as the positive electrode plate 14. The negative electrodematerial mixture can be prepared by mixing a negative electrode activematerial with a suitable dispersion medium. The negative electrodeactive material layer 17 can be formed by applying the negativeelectrode material mixture on one side or both sides of the negativeelectrode current collector 16, followed by drying and rolling. Thenegative electrode material mixture may contain additives such as abinder, a conductive agent, and a thickener, as necessary.

When a high capacity material such as silicon or tin is used as anegative electrode active material, the negative electrode activematerial layer 17 can be formed by depositing this high capacitymaterial on the negative electrode current collector 16 by a vacuumprocess. Specifically, a vacuum process such as a vapor depositionmethod, a sputtering method, or a CVD (chemical vapor deposition) methodcan be employed. Among them, the vapor deposition method is desirablefrom the viewpoint of forming the negative electrode active materiallayer 17 efficiently. As the vapor deposition method, either of electronbeam vapor deposition or resistance heating vapor deposition can beemployed. In the case where vapor deposition of an oxide and/or anitride of a high capacity material is performed, the oxide and/or thenitride can be used as an evaporation material. Furthermore, reactivedeposition may be performed by introducing oxygen gas, nitrogen gas, orionized or radicalized oxygen or nitrogen gas above the evaporationsource while evaporating the high capacity material.

It should be remembered that the positive electrode plate 14 that shouldconstitute the outermost of the electrode assembly 30 must have aportion consisting of the positive electrode current collector 12.Likewise, it should be remembered that the negative electrode plate 18that should constitute the winding start portion of the electrodeassembly 30 must have a portion consisting of the negative electrodecurrent collector 16, to which the negative electrode lead 34 must beattached.

The positive electrode plate 14 and the negative electrode plate 18 thathave been fabricated by the above methods are each wound around a feedroll and subjected to the next step. The separators 15 are previouslycut to a desired width, wound around feed rolls, and subjected to thenext step.

Next, a step of forming the electrode assembly 30 by combining thepositive electrode plate 14, the separators 15, and the negativeelectrode plate 18 is performed (Step S2). Specifically, as shown inFIG. 7, the positive electrode plate 14, the separators 15, and thenegative electrode plate 18 are fed from the feed rolls 44 to 47 tolamination rolls 48. The positive electrode plate 14, the separators 15,and the negative electrode 18 are laminated one another between thelamination rolls 48, and thus the unwound electrode assembly 30 isformed. In order to obtain the wound electrode assembly 30 having thestructure shown in FIG. 3, the positive electrode 14, the separators 15,and the negative electrode plate 18 are each cut to a predeterminedlength immediately before the lamination rolls 48. Sufficient tension isapplied to the positive electrode plate 14, the separators 15, and thenegative electrode plate 18 to prevent so-called winding misalignment.

Next, a step of winding the electrode assembly 30 is performed (StepS3). Specifically, as shown in FIG. 7, the electrode assembly 30 iswound around a winding core 36.

Furthermore, the elastic member 21 is placed on the positive electrodecurrent collector 12 (Step S4) while performing the winding step. Thatis, the step of placing the elastic member 21 is performed during thewinding step. Specifically, as shown in FIG. 7, a component feedingmachine 37 is placed between the lamination rolls 48 and the windingcore 36 on the path for conveying the electrode assembly 30, and theelastic member 21 is put on the positive electrode current collector 12using the component feeding machine 37. After the winding step starts,when the portion of the positive electrode current collector 12 thatshould constitute the outermost of the electrode assembly 30 reaches theeffective area of the component feeding machine 37 (beneath the machine37 in the present embodiment), an actuator for driving the winding core36, the conveyor rollers 49, the lamination rolls 48, etc. istemporarily stopped. The elastic member 21 is previously held in thecomponent feeding machine 37. The elastic member 21 is moved from thecomponent feeding machine 37 onto the positive electrode currentcollector 12, with the position of the positive electrode currentcollector 12 checked with a detector such as a CCD (charge-coupleddevice) camera. The elastic member 21 can be attached to the positiveelectrode current collector 12 with an adhesive tape, an adhesive agent,or the like. The step of winding the electrode assembly 30 and the stepof placing the elastic member 21 are performed alternately so that theelastic members 21 are incorporated into both of the corner portions 31of the wound electrode assembly 30.

The timing to perform the step of placing the elastic member 21 on thepositive electrode current collector 12 is not limited to that shown inFIG. 7. For example, the step of placing the elastic member 21 can beperformed between the step of preparing the positive electrode plate 14,the separators 15, and the negative electrode plate 18 and the step ofwinding the electrode assembly 30. Specifically, the elastic member 21can be placed on the positive electrode current collector 12 between thefeed rolls 47 and the lamination rolls 48 on the conveyor path. In thecase where the outermost is formed of the negative electrode currentcollector 16, the elastic member 21 can be placed on the negativeelectrode current collector 16. Furthermore, a method in which theelastic member 21 is placed on the separator 15 also may possibly beemployed. That is, the elastic member 21 can be placed on the positiveelectrode plate 14, the negative electrode plate 18, or the separator 15before the end of the winding step so that the elastic member 21 isincorporated inside the outermost of the wound electrode assembly 30 andinto at least one of the corner portions 31 of the electrode assembly 30in the winding direction.

Once the elastic member 21 is placed on the positive electrode currentcollector 12, the winding core 36, the conveyor rollers 49, etc. arerotated again to wind up the electrode assembly 30 (Step S5). When theelectrode assembly 30 is wound up, the winding end portion 32 is fixedwith the adhesive tape 19 (Step S6).

After the winding end portion 32 is fixed, the electrode assembly 30 ispressed into a predetermined thickness (Step S7). Thereby, a flat shapeis imparted to the electrode assembly 30. The electrode assembly 30 isaccommodated in the battery case 11 (or 41) (Step S8). After anelectrolytic solution is poured into the battery case 11, the batterycase 11 is sealed (Steps S9 and S10). Thereby, the battery 10 having theelectrode assembly 30 in which the elastic members 21 are incorporatedinto the corner portions 31 is obtained. The press working may beomitted.

By the way, in order to secure the space for absorbing the expansion ofthe electrode assembly, it seems to be a good idea to wind the electrodeassembly with reduced tension applied thereto. If reduced tension isapplied to the electrode assembly, however, so-called “windingmisalignment” is more likely to occur. The “winding misalignment” is aphenomenon that the positions of the positive electrode plate, thenegative electrode plate, and the separator respectively are misalignedwith their design positions. The occurrence of the “windingmisalignment” causes a decrease in yield and a decrease in productivity.

Furthermore, as a method for securing the space for absorbing theexpansion of the electrode assembly, it seems to be a good idea toinsert a kind of jig into the electrode assembly during the winding stepinstead of using “the porous elastic member 21” when the electrodeassembly is wound. However, such a method is not practical inconsideration that the electrode assembly is wound at a very high speed.It is difficult to insert a jig into the electrode assembly during thewinding step unless the winding speed is significantly slowed down,although it depends on the design conditions such as the number ofwinding turns. Furthermore, the possibility that the components such asthe separator are damaged when the jig is pulled out of the woundelectrode assembly cannot be denied.

In contrast, according to the method of the present embodiment, there isno need to reduce the tension applied to the electrode assembly 30, norto remove the elastic member 21 later. Therefore, this method is lesslikely to cause a decrease in yield and a decrease in productivity. Thecomponents such as the separator 15 are almost free from damage. Ofcourse, the present invention does not preclude the later removal of theelastic member 21 from the wound electrode assembly 30.

It should be noted that, in the case where the electrode assembly iswound in a cylindrical shape, the problem of an increase in thickness isless likely to occur than the case of a flat-shaped electrode assembly.This is because the electrode assembly having a cylindrical shape ismore resistant to deformation due to its own binding than theflat-shaped electrode assembly. However, this does not preclude theapplication of the present invention to cylindrical-shaped lithium ionsecondary batteries.

(Modification)

The elastic member 21 that has been described in the embodiment mayextend to the position where it overlaps the end portion of the positiveelectrode active material layer 13. With such a structure, the followingeffects are obtained. As known to those skilled in the art, the negativeelectrode active material layer generally has a larger area than thepositive electrode active material layer in plan view. Furthermore, inorder to enhance the safety of a lithium ion secondary battery,conventionally, an insulating tape made of, for example, polyphenylenesulfide is placed, at the end portion of the positive electrode activematerial layer, between the positive electrode plate and the separatorso as to cover the positive electrode active material layer or betweenthe negative electrode plate and the separator so as to cover thenegative electrode active material layer. This insulating tape isdifferent from the elastic member 21 of the present invention. However,if the elastic member 21 of the present invention and the conventionalinsulating tape in the lithium ion secondary battery are integrated intoa single member, not only the number of components can be reduced butalso the number of production processes may be able to be reduced.

As shown in FIG. 8, an elastic member 21A according to a modificationextends in the longitudinal direction LD of the electrode assembly 30 tothe position where it overlaps the end portion 13 e of the positiveelectrode active material layer 13. At that position where the elasticmember 21A overlaps the end portion 13 e, the elastic member 21A isinterposed between the positive electrode active material layer 13 andthe separator 15. If the thickness of the elastic member 21A is adjustedappropriately in the longitudinal direction LD, a thinner portion of theelastic member 21A can be inserted between the positive electrode activematerial layer 13 and the separator 15 without difficulty. The elasticmember 21A may be interposed between the negative electrode activematerial layer 17 and the separator 15. With such a structure, not onlythe insulation between the positive electrode current collector 12 andthe negative electrode active material layer 17 can be ensured, but alsodefects near the end portion 13 e of the positive electrode activematerial layer 13, for example, a short circuit caused by theprecipitation of lithium, can be surely prevented. The increase in thenumber of components caused by the addition of the elastic member 21Aalso can be avoided.

Furthermore, as shown in FIG. 9, the elastic member 21 may beincorporated not only into the corner portions 31 but also into the flatportions of the electrode assembly 30. In the modification shown in FIG.9, the elastic member 21 having a length corresponding to one turn ofthe electrode assembly 30 is incorporated inside the outermost of theelectrode assembly 30.

EXAMPLES Example 1

100 parts by weight of LiCoO₂, 2 parts by weight of acetylene black, 3parts by weight of polyvinylidene fluoride, and a proper amount ofN-methyl-2-pyrrolidone were mixed to obtain a positive electrodematerial mixture. This positive electrode material mixture was appliedto a positive electrode current collector made of a strip-shapedaluminum foil with a thickness of 15 μm. The positive electrode materialmixture was not applied to the portion of the positive electrode currentcollector that should constitute the outermost of the electrodeassembly. After the applied positive electrode material mixture wasdried at 110° C. for 5 minutes, rolling was performed three times.

A positive electrode lead made of aluminum was fixed to the positiveelectrode current collector by spot welding. In order to prevent aninternal short circuit, an insulating adhesive member made ofpolypropylene was attached to the positive electrode current collectorso as to nip the positive electrode current collector. Thus, a positiveelectrode plate with a width of 35 mm, a length of 460 mm, and athickness of 0.14 mm was prepared.

100 parts by weight of flaky graphite, 1 part by weight in terms ofsolid content of styrene-butadiene rubber in the form of an aqueousdispersion, 1 part by weight of carboxymethyl cellulose as a thickener,and a proper amount of water were mixed to obtain a negative electrodematerial mixture. This negative electrode material mixture was appliedto a negative electrode current collector made of a strip-shaped copperfoil with a thickness of 10 μm. After the applied negative electrodematerial mixture was dried at 110° C. for 30 minutes, rolling wasperformed.

A negative electrode lead made of nickel was fixed to the negativeelectrode current collector by spot welding. In order to prevent aninternal short circuit, an insulating adhesive member made ofpolypropylene was attached to the negative electrode current collectorso as to nip the negative electrode current collector. Thus, a negativeelectrode plate with a width of 36 mm, a length of 450 mm, and athickness of 0.14 mm was prepared.

Next, an electrode assembly was formed by combining the positiveelectrode plate and the negative electrode plate via separators, and theelectrode assembly was wound. Porous elastic members were incorporatedinto the two corner portions of the electrode assembly. The winding endportion was fixed with an adhesive tape. Thereby, an electrode assemblyhaving the structure that has been described with reference to FIG. 3was obtained. As the separator, a microporous polyethylene membrane witha thickness of 16 μm was used. As the elastic member having porosity, asquare bar of polyethylene foam with dimensions of 1 mm×1 mm×36 mm(special polyethylene foam manufactured by Fujishika Industrial Co.,Ltd. (a closed-cell type foam with an expansion ratio of 30 times) witha thickness of 1 mm) was used. An adhesive agent (an acrylic adhesiveagent) was used to attach the elastic member to the positive electrodecurrent collector. As the adhesive tape for fixing the winding endportion, a tape having a 20 μm-thick substrate made of polyphenylenesulfide and a 5 μm-thick adhesive agent layer made of butyl acrylate wasused.

Subsequently, a flat shape was imparted to the electrode assembly bypress working. The electrode assembly thus obtained was accommodated ina battery case made of a laminated packaging material. As the laminatedpackaging material, a material in which polypropylene films with athickness of 10 μm were laminated on both sides of an aluminum foil witha thickness of 100 μm was used.

After the electrode assembly was accommodated in the battery case, theelectrode assembly was vacuum dried at a temperature of 85° C. for 2hours. After the drying, the water content of the electrode assembly wasmeasured with a Karl-Fischer moisture meter. It was confirmed that thewater content of the electrode assembly was 100 ppm or less.

LiPF₆ as an electrolyte was dissolved at a concentration of 1.0mol/liter in a mixed solvent containing ethyl carbonate and ethyl methylcarbonate in a volume ratio of 1:2. Thereby, a non-aqueous electrolyticsolution was obtained. The non-aqueous electrolytic solution was pouredinto the battery case, and the battery case was sealed by heat welding.In this way, a prismatic lithium ion secondary battery having a batterycapacity of 800 mAh (design value) was obtained.

Comparative Example 1

A prismatic lithium ion secondary battery was produced in the samemanner as in Example 1, except that the elastic member was notincorporated thereinto.

Comparative Example 2

A prismatic lithium ion secondary battery was produced in the samemanner as in Example 1, except that as an adhesive tape for fixing thewinding end portion, a tape having the structure shown in FIG. 10 wasused. An adhesive tape 50 shown in FIG. 10 includes a substrate 51 andadhesive agent layers 52 provided on both ends of the substrate 51. Withthe adhesive tape 50, the folded portion 53 is unfolded and therebyallows the electrode assembly to expand. That is, Comparative Example 2is a replication of Example 4 of JP 2006-302801 A.

<Measurement of Initial Thickness>

Three batteries were produced for each of Example 1, Comparative Example1, and Comparative Example 2, and the initial thickness of each batterywas measured by the following method. Each battery had a thickness of4.9 mm as a design value immediately after it was assembled.

Specifically, each battery was charged at a constant current of 800 mA(1.0 CmA) in an atmosphere of 20° C. until the battery voltage reached4.2 V, and further charged at the constant voltage until the currentdropped to 40 mA (0.05 CmA). It took about 2 hours to charge thebattery. After the charging, the thickness of the battery was measuredwith a thickness gauge at 9 points across the flat surface of thebattery, and the average of the values of the 9 points was calculated asthe “initial thickness of the battery”. Table 1 shows the results.

TABLE 1 Comparative Comparative Item Example 1 Example 1 Example 2Whether an elastic Incorporated Not Not member is incorporated intocorner incorporated incorporated or not portions Whether an adhesivetape Not Not Specially is specially processed processed processedprocessed or not Initial thickness of battery 5.22 mm 5.32 mm 5.21 mm5.20 mm 5.29 mm 5.29 mm 5.18 mm 5.25 mm 5.25 mm Average thickness 5.20mm 5.29 mm 5.25 mm

When focusing on the average thicknesses, the batteries of Example 1 andComparative Example 2 had smaller initial thicknesses than that ofComparative Example 1. Comparative Example 2, however, included abattery showing a good result (5.21 mm) comparable to that of Example 1and a battery showing a result (5.29 mm) close to the average thicknessof Comparative Example 1. In contrast, all the batteries of Example 1showed good results. This means that the variations in the obtainedeffect were smaller in Example 1. According to Example 1, the expansionof the electrode assembly was absorbed uniformly in both of the cornerportions, and presumably, this reduced the variations in the initialthickness.

INDUSTRIAL APPLICABILITY

The present invention is useful for flat-shaped lithium ion secondarybatteries, particularly for thin lithium ion secondary batteries aspower sources for small electronic devices.

The invention claimed is:
 1. A lithium ion secondary battery having aflat shape, the battery comprising: an electrode assembly including apositive electrode plate, a negative electrode plate, and a separator,the electrode assembly being wound around an axis into a flat shapehaving a curved end defining a corner portion; and an elastic memberarranged inside the electrode assembly and into at least the cornerportion of the electrode assembly such that at least a portion of theelastic member is positioned between at least a portion of one of thepositive electrode plate and the negative electrode plate and at least aportion of the separator in a direction perpendicular to the axis, theelastic member being made of a material having porosity, and wherein theelastic member is made of at least one material selected from the groupconsisting of polyethylene, polypropylene, and polyurethane.
 2. Thelithium ion secondary battery according to claim 1, wherein the elasticmember is arranged into only the corner portion.
 3. The lithium ionsecondary battery according to claim 1, wherein the flat shape has twocurved ends defining two corner portions, and the elastic member isarranged into each of the two corner portions.
 4. The lithium ionsecondary battery according to claim 1, wherein the positive electrodeplate includes a positive electrode active material layer and a positiveelectrode current collector, the negative electrode plate includes anegative electrode active material layer and a negative electrodecurrent collector, and the at least a portion of the elastic member ispositioned between one of the positive electrode current collectorwithout the positive electrode active material layer provided thereonand the negative electrode current collector without the negativeelectrode active material layer provided thereon and the at least aportion of the separator in the direction perpendicular to the axis. 5.The lithium ion secondary battery according to claim 1, wherein theelastic member is formed of a foam of the material.
 6. A method forproducing a lithium ion secondary battery having a flat shape, themethod comprising: a step of preparing a positive electrode plate, aseparator, and a negative electrode plate; a step of forming anelectrode assembly by combining the positive electrode plate, theseparator, and the negative electrode plate, and winding the electrodeassembly around an axis into a flat shape having a curved end defining acorner portion; a step of placing an elastic member, made of a materialhaving porosity, on the positive electrode plate, the negative electrodeplate, or the separator before the end of the winding step so that theelastic member is arranged inside the electrode assembly and into atleast the corner portion of the electrode assembly such that at least aportion of the elastic member is positioned between at least a portionof one of the positive electrode plate and the negative electrode plateand at least a portion of the separator in a direction perpendicular tothe axis; and a step of fixing a winding end portion of the electrodeassembly, and wherein the elastic member is made of at least onematerial selected from the group consisting of polyethylene,polypropylene, and polyurethane.
 7. The method for producing a lithiumion secondary battery according to claim 6, wherein the elastic memberis arranged into only the corner portion.
 8. The method for producing alithium ion secondary battery according to claim 6, wherein the flatshape has two curved ends defining two corner portions, and the elasticmember is arranged into each of the two corner portions.
 9. The methodfor producing a lithium ion secondary battery according to claim 6,wherein the positive electrode plate includes a positive electrodeactive material layer and a positive electrode current collector, thenegative electrode plate includes a negative electrode active materiallayer and a negative electrode current collector, the at least a portionof the elastic member is positioned between one of the positiveelectrode current collector without the positive electrode activematerial layer provided thereon and the negative electrode currentcollector without the negative electrode active material layer providedthereon and the at least a portion of the separator in the directionperpendicular to the axis, and in the step of placing the elasticmember, the elastic member is placed on a portion of the positiveelectrode plate consisting of the positive electrode current collectoror on a portion of the negative electrode plate consisting of thenegative electrode current collector.
 10. The method for producing alithium ion secondary battery according to claim 6, wherein the step ofplacing the elastic member is performed between the preparing step andthe winding step or during the winding step.